1. Field of the Invention
The present invention relates to an optical module, particularly relates to an optical module used in a high speed region.
2. Description of Related Art
For a modulator in optical communication, an optical function element mainly utilized in a high speed region exceeding the transmission rate of 10 Gbits/s and excellent in high-speed optical modulation characteristics and dispersion-tolerant characteristics has been mounted in a casing separate from a light emitting element remarkably different in a coefficient of thermal expansion.
A light emitting element represented by a semiconductor laser diode is mounted in a package made of substantially the same material as that of the light emitting element in a coefficient of thermal expansion in a state in which it is airtightly sealed. A beam output from the light emitting element is outgoing from a fiber via a condenser lens.
Similarly, an optical function element in which an optical waveguide provided with electro-optical effect represented by lithium niobate (LiNbO3) is formed is also mounted in a package having substantially the similar coefficient of thermal expansion to a coefficient of thermal expansion of the optical function element in a state in which it is airtightly sealed. As for the incoming and outgoing radiation of the optical function element, there are a case that the optical function element is connected to a fiber via a lens and a case that the optical function element is directly connected to a fiber by an optical adhesive.
Generally, an optical function element has dependency upon a direction of polarization. Therefore, to make a beam incident upon the optical function element via a fiber, a polarization maintaining fiber is utilized because polarization is required to be held. If a package for a light emitting element and a package for the optical function element are separate and they are connected, a polarization maintaining fiber on the side of the light emitting element package and a polarization maintaining fiber on the input side of the optical function element package are required to be connected via a connector or to be spliced to each other.
In a conventional type configuration that a light emitting element and an optical function element are housed in separate packages, space for connecting fibers between the packages is required. Besides, as the element and the device are connected via the fibers, maintaining a plane of polarization, the high-priced polarization maintaining fibers and the alignment of polarization are required and the cost is increased. Therefore, the integration of the light emitting element and the optical function element in the same package which contributes to miniaturization and the reduction of the cost is expected.
In a Patent Reference 1 (Japanese Laid-Open Patent Publication No. Hei 11-218648 (No Family)), an optical composite module in which a light emitting element and an optical function element are housed in separate packages and the respective packages are welded is described. In the optical composite module, a polarization maintaining fiber between the light emitting element and the optical function element can be omitted.
Besides, in a Patent Reference 2 (Japanese Laid-Open Patent Publication No. 2001-272572 and its counterpart: US-2002-009265A1), a method of packaging an optical function element module having a structure that an input terminal and an output terminal of an optical function element are fixed to fibers and further the fibers are fixed to a package, is described.
In a Patent Reference 3 (Japanese Laid-Open Patent Publication No. 2000-150745 (No Family)), a package is described which houses an optical semiconductor device having a special radiation plate inserted to be set in its base member made of an iron-nickel alloy or an iron-nickel-cobalt alloy.
In a Patent Reference 4 (Japanese Laid-Open Patent Publication No. 2001-015635 (No Family)), a package is described which houses an optical semiconductor device having a cooling member (an electronic cooling element) inserted to be set in its base member made of an iron-nickel-cobalt alloy or a ceramic such as aluminum oxide sintered body.
Further, in a Patent Reference 5 (Japanese Laid-open Patent Publication No. 2003-273437 (No Family)), a package for an optical semiconductor module is described in which the periphery of a cutout provided to a ceramic base and an electronic cooler are connected.